Tungsten carbide drill bits have been used for drilling into soil and also hard rock, and have been available for many years, have been effective for their routine usage, but they do have a tendency to become dull, or fracture, particularly when drilling into hard rock.
Current tungsten carbide bits tend to dull down very fast in very hard, high silica ground, with a compressive strength of 30,000-60,000 psi.
The drill settings, when using tungsten carbide bits are generally set at 30 Bar or more for the thrust and torque for the first hole or two, then the thrust is increased to the maximum on the drill of approximately 50 Bar, to drive the dull bit into the rock.
Tungsten carbide bits cannot drill efficiently at very low thrust and torque settings in hard rock drilling applications, as the carbide inserts dull down severely, and stop the drill bit from penetrating the ground. Very high thrust and torque settings, sometimes up to 50 Bar, are required when drilling with these types of carbide bits.
The prior art and use of tungsten carbide insert bit designs has proven to be marginally satisfactory and the design and use of polycrystalline diamond (PCD) type inserts has substantially improved the performance of percussion drill bits. It has taken many years to perfect the design of the PCD bits, as a replacement for carbide tips, particularly since in the PCD bit structure, the range for the dimensions of the inserts, the bit bodies, and the drill settings, are very limited and narrow. Inserts used in prior art could have a very wide range of dimensions, and still be considered acceptable.
Prior art patents or publications include U.S. Pat. No. D574,403 and U.S. Publication No 2009/0184564. Other prior art patents relating to this technology include U.S. Pat. No. 5,944,129, in addition to U.S. Pat. No. 3,788,409.
As further commented, carbide bits operate at very high thrust and torque, usually in the range of 50 Bar or above for thrust and 30 Bar or above for torque, and their insert tip length is shorter, and there are many more inserts included in the bit body, than are required or needed in the PCD type of percussion drill bit. Carbide bits dull down fairly fast, particularly in abrasive rock, thus slowing down the penetration rate, and require an elevation of thrust and torque, in order to achieve any continuous drilling. Furthermore, it has been experienced that carbide bits wear down 10 to 15 times faster than a PCD drill bit, and are 30% to 50% slower in drilling penetration rates.
Through testing, it has been determined that the normal time to drill a 50 hole round within hard rock, using the carbide drill bit, was in excess of three hours, but utilizing a PCD insert drill bit, could achieve the same drilling of a 50 hole round in a time between about one hour and one and a half hours. PCD bits complete a drilling operation using 70% less thrust and 60% less torque.
The usage of PCD bits, in comparison to carbide bits, found that the PCD percussion drill bits were more productive; at far less thrust and torque parameters, than the carbide bits.
Furthermore, because there are fewer PCD bits to change out due to little or no dulling, during prolonged usage, there is a lesser chance of injuries to the miner, and workers handling such equipment.
Because of the reduction in thrust and torque forces required, up to approximately the 70% range, when drilling with PCD insert drill bits in comparison to the use of carbide bits, there is less heat buildup in the drill steel sections and there is a less torsional forces exerted on the steel sections that can cause fatigue and failure, and therefore, the drill steel section life of the entire equipment is vastly improved using the PCD insert bits.
Furthermore, because of the very low thrust and low torque that is used when drilling with PCD double chisel insert bits or modified double chisel bits, as has been found through experimentation, there is less stress on the hydraulic components of the machinery, that there is less wear on the drilling machine and there is less heat buildup in the drill steel sections, as explained when utilizing the PCD bits in comparison with carbide bits.
Finally, it was observed during experimentation, that the powder crews, when depositing their explosives within the drilled holes, that there was a very significant reduction of cuttings left in the hole that were drilled with the PCD bits as compared with the carbide bit drilled holes, and this resulted in significantly less time cleaning the holes with a spoon, during the performance of the crew's duties.
It can be seen that when compared to the prior art style of carbide bits, that the PCD bits have far more advantages, which have been identified during experimentation and testing, at very significant cost reduction advantages for the mine. Production is nearly doubled with the use of the PCD drill bit. The bit cost per foot of hole is reduced, drilling time of the holes is significantly reduced and maintenance costs are minimized.